The mobile radio communication system recently spreads. In the portable radio device, or the like, for example, the user anticipates further reduction in size, weight and cost of the device. Also, the portable radio device capable of meeting a plurality of communication systems that make transmission/reception in a plurality of different frequency bands is now investigated. Thus, the user expects the device to handle the frequency bands of a plurality of communication systems by one antenna unit. As a result, a smaller size, a lower cost attained by the reduction in the number of articles and the assembling man-hours, or a wider frequency characteristic to be secured, and so forth are required of the antenna unit that is incorporated into the portable radio device. However, normally the antenna unit tends to have a narrower bandwidth commonly when the size of such antenna unit is reduced smaller.
Meanwhile, as the antenna unit used in the portable radio device in the prior art, e.g., the mobile phone, the antenna unit shown in FIG. 1 is known. In this case, FIG. 1 is an appearance view of the antenna unit showing the state that a whip antenna 202 is pulled out from a conventional mobile phone 200.
This mobile phone 200 has a telescopic antenna unit. The whip antenna 202 starts operation when such whip antenna 202 is pulled out from a housing 201. Also, a helical antenna 203 starts operation when the whip antenna 202 is pushed into the housing 201.
Meanwhile, according to this telescopic antenna unit, the helical antenna 203 of this antenna unit is always protruded from the housing 201 of the mobile phone 200 to the outside, and thus the presence of such protruded portion causes inconvenience to the user upon carrying and operating the phone. In particular, the small-size mobile phone 200 is often put into the user's breast pocket. For this reason, it is possible that the antenna may hit on various things during carried in the rocket, and a physical strength of the antenna cannot be satisfactorily maintained.
Therefore, in order to overcome disadvantages such as troublesome, incomplete physical strength, and the like, the built-in antenna unit whose antenna element is built in the interior of the main body of the portable radio device is known, as disclosed in JP-A-2000-349526, for example.
However, since this built-in antenna unit is arranged in vicinity of the liquid crystal screen, the board, the speaker, etc. which are constituting the portable radio device, such antenna unit is easily affected by these parts. It is known that normally such antenna unit operates in the narrower bandwidth.
For this reason, in many cases the wider bandwidth is realized by providing the matching circuit to the preceding stage of the feeding portion and then adjusting the impedance matching.
However, in the case where the wider bandwidth is realized by the matching circuit, a space in which the matching circuit is mounted must be kept on the printed board in the housing. Thus, there is a possibility that an increase in the mounting space on the printed board and an increase in the number of articles are brought about.
Also, normally the telescopic antenna unit in the prior art is constructed such that such device is unbalancedly fed to flow the antenna current through the housing of the portable radio device. In such unbalanced antenna unit, it is known that the antenna gain is degraded by the influence of the user's hand, and so on when the user holds the portable radio device to use.
Also, this portable radio device is regulated by the law based on SAR (Specific Absorption Rate), and it is requested to suppress the SAR value below a predetermined value. In such portable radio device, normally the state in which the user puts the portable radio device to his or her ear to contact closely to the head of the human body and speaks upon the phone, and so forth, for example, are considered as “the state in which the SAR value is increased”. Thus, according to the regulation by the law, further reduction in the SAR value is driven by necessity.
Therefore, as approaches of reducing the SAR value during the speaking in the prior art, three approaches described in the following are considered, for example.
First, it is known that the SAR value can be reduced by increasing an air clearance between the antenna unit and the head of the human body. Since normally the earpiece portion comes closest to the ear during the speaking, mainly a distance between the earpiece portion and the ear should be extended herein. However, in order to increase this air clearance, the antenna element must be positioned away from the head of the human body during the speaking by enlarging the housing of the portable radio device, for example. Thus, there is a possibility of causing an increase of the device in size.
Second, it is known that the SAR value can be reduced by reducing a set value of the maximum sending power. However, there is a possibility that the communication quality in the weak electric field area cannot be kept when the set value is reduced.
Third, as disclosed in JP-A-11-307144, the SAR value can be reduced by increasing an air clearance between a peak point of the antenna current (a point at which the largest antenna current is generated) and the head of the human body. This approach is available in the configuration in which the peak point is separated away from the head of the human body during the speaking. However, in the antenna unit having the configuration set forth in this publication, the peak point of the antenna current is only one and thus the peak point comes close to the head of the human body according to a mode of use of the user. Thus, there is a possibility of increasing the SAR value.
Here, the SAR value as the object of the law regulation is the numerical value used when the radio wave is radiated from the antenna unit provided to the portable radio device. Since there is no need to take account of such value upon receiving the radio wave, only the transmission band should be checked.
Next, FIG. 2 is an explanatory view showing the radiation directivity when a parasitic element 213 is brought close to an antenna element 212.
In FIG. 2, the antenna element 212 is a monopole antenna whose effective length is a half wavelength (λ/2) of a transmitted wavelength (λ), and is fed from a feeding portion 214. In contrast, the parasitic element 213 is formed of a wire, or the like, for example, whose length is shorter than the half wavelength (λ/2), and is arranged in the proximity of the antenna element 212.
In the case of an antenna unit 210 having such configuration, it is known that the parasitic element 213 operates as a waveguide element and thus the radiation directivity of the antenna unit 210 becomes strong in the +X direction rather than the −X direction.
As explained above, normally the bandwidth of the above conventional antenna unit is liable to become narrower when the reduction of the antenna unit in size is advanced.
Also, in the case of the above conventional telescopic antenna unit, there are the problems that the protrusion of the antenna unit from the portable radio device causes inconvenience upon carrying and operating the phone, and in addition the physical strength cannot be kept.
Also, since the above conventional built-in antenna unit disclosed in above JP-A-2000-349526 is arranged in vicinity of the liquid crystal screen, the board, the speaker, etc. constituting the portable radio device, normally the bandwidth tends to become narrow.
Also, in the above conventional unbalanced antenna unit, there is the problem that the antenna gain is degraded by the influence of the user's hand when the user holds the portable radio device to use.
Also, in the above conventional portable radio device, the increase in size of the device is brought about when the air clearance between the antenna unit and the head of the human body is increased during the speaking state to reduce the SAR value.
Also, in the above conventional portable radio device, there is the disadvantage that the communication quality is degraded in the weak electric field area when the set value of the maximum sending power is decreased to reduce the SAR value.
Here, current distributions 222 to 252 and current peak points 221 to 251 in respective antennas of a half-wave monopole antenna 220, a one-wave monopole antenna 230, a half-wave dipole antenna 240, and a one-wave dipole antenna 250 will be explained with reference to FIGS. 3(A) to (D) hereunder.
As shown in these Figures, it is appreciated in these monopole antennas and dipole antennas that, in the case of the half wavelength, the current peak points 221 to 251 are positioned in one center point of the antenna element respectively and, in the case of the one wavelength, the current peak points are scattered into two points respectively.
On the contrary, in the antenna unit set forth in above JP-A-11-307144, there are the problems that the peak point of the antenna current is only at one location, and the peak point comes close to the head of the human body according to change in the using situation of the user, and thus the SAR value is apt to increase.
Therefore, the present invention has been made in light of above circumstances, and it is an object of the present invention to provide an antenna unit and a portable radio device capable of realizing a wider band and realizing a good antenna performance by controlling the radiation directivity and in addition reducing SAR.